02 mn1789eu10mn 0002 overload handling

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Overload handling Siemens

MN1789EU10MN_0002 2004 Siemens AG

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Contents

1 Introduction 32 BSC overload 52.1 BSC overload conditions 62.2 BSC overload reactions 72.3 Timer control of overload reduction 82.4 Traffic reduction algorithm 103 MSC overload 154 BTS overload 174.1 BTS overload conditions 184.2

Traffic reduction algorithm 19

4.3 Simultaneous BTS and BSC overload 214.4 Parameters for overload handling 224.5 Preemption, queuing 23

Overload handling


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1 Introduction

The aim of the overload control mechanism is to preserve the traffic-handlingcapabilities of the mobile base station system (BSS) under adverse conditions.

The overload control mechanism is implemented in the BSC and consists of trafficdetection mechanism and defensive actions.

Traffic detection mechanismconsists of overload messages coming either fromthe various parts of the network (BTS and/or MSC) of from the BSC itself (overloaddetection).

Defensive actionsaimed at reducing both mobile originating (MOC) and/or mobileterminating (MTC) calls (overload handler); the strategy is here presented and followsthe GSM recommendation 08.08.

GSM rec 08.08gives the following definition of the general overload handlingstrategy: The overload causing traffic is reduced in several timer guarded steps untilthe overload is prevented. Otherwise, if for some time no overload is indicated, thetraffic is increased again in timing steps until full load has been resumed.

The time-out values of the timers, number of steps, amount and type of trafficreduced in each step, overload recognition and threshold parameters are allconsidered implementation dependent and have not been specified.

Essentially there are two methods to reduce overload:

reducing the mobile terminating traffic by discarding paging messages,

reducing the mobile originating traffic by barring the access to specific cells.

Access barring is done using the access classes specified by GSM (refer to GSM08.08).

There are 10 access classes (0...9) related to normal subscribers. The access class

to which a subscriber belongs is derived from his IMSI.Furthermore, there are 5 classes (11...15) assigned to special high prioritysubscribers (e.g. police, PLMN operator, ...).

To reduce overload in moderate way, incoming traffic is not completely rejected, butreduced by several steps of escalation controlled by timers.


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2 BSC overload


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2.1 BSC overload conditions

The following overload conditions are recognized:

Tx buffers for CCS7 links are congested

Buffering is a CCS7 level 2 function handled by PPXX (PPCC). Congestion mayoccur because of high traffic volume or frequent retransmission (in case of badtransmission quality).

Percentage of busy level 3 radio registers to handle incoming call requestsexceeds a hard-coded threshold of 90 %

For dedicated transactions (e.g. MOC, MTC, SMS, incoming handover, location

update), a (release-dependent) number of level 3 call processing transactionregisters is available.

Real time TDPC load exceeds the threshold OVLSTTHR

The database parameter OVLSTTHR determines the start of TDPC real timeprocessor overload, OVLENTHR gives the end of this overload condition.

Note: This overload condition is only detected if the database flag BSCOVLH is set to"TRUE".

Lack of TDPC memory detected

If the memory occupation of TDPC exceeds 70 %, the overload condition starts. Forvalues below this hard-coded threshold, the overload condition has ended.Oscillations are avoided through the usage of timer T18.

Real time processor load of PPXX exceeds 70 %

The threshold for PPXX processor overload is hard-coded to 70 %.

This overload condition is not evaluated for PPLD modules.

Lack of PPXX memory resources

If the usage of the PPXX memory goes above the hard-coded value of 70 %, thisoverload condition is detected.

This overload condition is not evaluated for PPLD modules.

Overflow in BSC paging queue

PAGING messages from MSC are queued in the BSC (length: 40 messages). Aftersuccessful delivery of PAGING COMMAND via Abis, the PAGING messages areremoved from the paging queue.


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2.2 BSC overload reactions

For all overload conditions (with the exception of "Overflow in BSC Paging Queue"),the overload actions taken by the BSC are

alarm message "BSC Overload Detected" (incl. specific overload cause value) issent to RC/LMT/event log file

BSSMAP OVERLOAD message is sent to MSC (with cause "processor overload")

mobile terminating traffic (PAGING messages from MSC) as well as

mobile originating traffic (CHANNEL REQUIRED messages from BTS) arediscarded.

For the overload cause "Overflow of BSC Paging Queue", neither mobile originatingnor mobile terminating traffic is reduced (directly).

However, the following alarm messages are issued:

"A-Interface Paging Overload" is sent to RC/LMT/event log file and

BSSMAP OVERLOAD message is sent to MSC.

Therefore, mobile terminating traffic is reduced indirectly by the MSC.


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2.3 Timer control of overload reduction

When the BSC detects an overload situation, it performs the following actions: start timers T2 and T18,

set overload level to "1",

send "OVERLOAD" message to MSC,

send "overload detected" message to RC/LMT/event log file.

Timer T2 is fixed to 2 sec.

If after the expiry of T2, the overload condition is still present, the variable"overload level" is increased by one step.

If the overload condition has disappeared (at expiry of T2), the variable isdecreased by one step.

After the overload level has reached the level "0" again and the timer T18 expires,the overload condition has ended. If T18 expires before the overload level "0" hasbeen reached it is restarted.

T2: Overload level de-/escalation timer

T18: Overload O&M alarm observation timer


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To MSC: OVERLOAD

Overload level = 1

Overload level = 2

If overload still present:

T 2 (2 sec) T 18

To O&M: overload detected

Overload

Detected

To MSC: OVERLOAD

If overload still present:To MSC: OVERLOAD

Overload level = 3

If overload NO more present:

Overload level = 2

If T 18 expires foroverload level > 0:

T 18 is restarted


Overload level = 1


Overload level = 0

Overload

End

To O&M: overload ceased

BSC

Fig. 1 Overload level management

The overload de-/escalation timer T2is fixed to 2 sec. If an overload condition isstill present after expiry of T2, the variable "overload level" is increased. If anoverload condition has disappeared after expiry of T2, the variable "overload level" isdecreased.

The overload observation timer T18is set in the BSC timer package:

DB Name Range Unit Meaning

BSCT18 0...254

Def: 60 HLFSEC.

HLFSEC, SEC5 Overload observationtimer


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2.4 Traffic reduction algorithm

Mobile originating traffic is reduced by discarding CHANNNEL REQUIRED messages(on Abis).

Mobile terminating traffic is reduced by discarding PAGING messages (from Ainterface).

The extent to which these messages are discarded depends on the current overloadlevel. The BSC administers only one overload level variable which determines theextent of discarding both CHANNNEL REQUEST and PAGING messages.

CHANNEL REQUIRED messages with the two following establishment causes arenotdiscarded:

"EMERGENCY CALL" and "ANSWER TO PAGING".


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OverloadLevel

"1" "2" "3" "4" "5" "6" "7" "8" "9" "10" %

0 0

1 X 10

2 X X 20

3 X X X 30

4 X X X X 40

5 X X X X X 50

6 X X X X X X 60

7 X X X X X X X 70

8 X X X X X X X X 80

9 X X X X X X X X X 90

10 X X X X X X X X X X 100


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2.4.1 Special algorithm: BSC paging queue overflow

If PAGING messages are discarded due to lack of space in the BSC paging queue

an alarm is sent to RC/LMT/event log file and

BSSMAP OVERLOAD message is sent to MSC.

In addition, the two following timers are started:

the BSC paging queue overload observation timer T1 (fixed at 1 sec) and

the BSC overload O&M alarm observation timer.

If during the runtime of T1, PAGING messages are discarded (again), T18 isrestarted (after expiry of T1) and a BSSMAP OVERLOAD message is sent to MSCagain.

If no PAGING messages are discarded during runtime of T1, only T1 is restarted andobservation continues.

If no PAGING messages are discarded during the runtime of T18 the alarm isceased.


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To MSC: OVERLOAD

If PAGING msg. discarded:

T 1 (1 sec) T 18

To O&M: overload detected

PAGING msg.

from MSC

discarded

To MSC: OVERLOAD

If PAGING msg. discarded:

If NO PAGING msg. discarded:

If PAGING msg. are

discarded, T1 and

T18 are restarted

Overload End

To O&M: overload ceased

BSC



If NO PAGING msg.

are discarded, only

T18 is restarted

To MSC: OVERLOAD

Fig. 2 Overload algorithm in case of BSC Paging Queue Overflow


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3 MSC overload

BSC overload reaction due to MSC overload is controlled with the database flagMSCOVLH.

MSC overload is signaled with the BSSMAP OVERLOAD message from MSC.

Reactions to MSC overload are the same as described above for BSC overload:

BSC sends an alarm message "MSC overload detected" to RC/LMT/event log file,

BSC starts to reduce mobile terminating traffic (i.e. systematically discardsPAGING messages) and

BSC starts to reduce mobile originating traffic (i.e. systematically discards

CHANNEL REQUIRED messages).


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4 BTS overload


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4.1 BTS overload conditions

The following overload conditions are recognized by the BSC:

OVERLOAD message (cause "CCCH overload") from BTS via Abis/RSL

In the BTS, two slots (i.e. "PAGING REQUEST" messages) for each paging groupare available. The OVERLOAD message is sent if PAGING COMMANDs (from Abis)cannot be placed in a paging queue.

DELETE INDICATION message from BTS via Abis/RSL

For each CCCH the BTS has an AGCH queue with 16 slots. How fast this is emptieddepends on the volume of paging messages and the amount of CCCH blocks

reserved for AGCH. The DELETE INDICATION message is sent by the BTS to theBSC if an IMMEDIATE ASSIGNMENT or IMMEDIATE ASSIGNMENT REJECTmessage could not be placed in the respective AGCH queue.

Abis LAPD signaling overload for PPLD

PPLD modules have layer 2 buffers for LAPD messages. In case of excessivesignaling volume or frequent re-transmission buffer congestion may occur. If the Abisoverload situation has been detected for a particular BTSM, the BSC regards all BTSbelonging to this BTSM as overloaded.

BTS detects RACH overloadIn case the percentage of busy RACH exceeds the threshold TCCCHLDI, the BTSsends CCCH LOAD INDICATION message to BSC (periodicity PCCCHLDI).

Note: No special overload actions are taken.


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4.2 Traffic reduction algorithm

BTS overload handling works similar to BSC overload handling by reducing mobileterminating traffic by discarding PAGING messages and mobile originating traffic bydiscarding CHANNEL REQUIRED messages.

The differences between BSC and BTS overload handling are:

BTS overload handling is restricted to the BTS affected

two different thresholds are administered for the discarding of CHANNELREQUIRED messages ("OVLD_LEVEL_UPLINK") and PAGING messages("OVLD_LEVEL_DOWNLINK"):

PCH overload (after OVERLOAD message from BTS) increments

OVLD_LEVEL_DOWNLINK only,AGCH overload (after DELETE INDICATION from BTS) incrementsOVLD_LEVEL_UPLINK only (Note: the second DELETE_INDICATION messagetriggers the first increment)

if PPLD Abis LAPD signaling overload is detected, both OVLD_LEVEL_UPLINKand OVLD_LEVEL_DOWNLINK are set to the max value ("10")

4.2.1 PCH overload

After the message OVERLOAD "CCCH Overload" has been received by the BSC

the alarm message "BTS overload detected" is sent to RC/LMT/ event log file,

the BSSMAP OVERLOAD message is sent to MSC,

the mobile terminating traffic is reduced by discarding PAGING messages for theaffected BTS based on the current value of OVLD_LEVEL_DOWNLINK (startingvalue for the variable after the first Abis overload message is "0").

4.2.2 AGCH overload

After the message DELETE INDICATION has been received by the BSC

the alarm message "BTS overload detected" is sent to RC/LMT/event log file,


the mobile originating traffic is reduced by discarding CHANNEL REQUIREDmessages for the affected BTS based on the current value ofOVLD_LEVEL_UPLINK (starting value for the variable after the first Abis overloadmessage is "0").


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4.2.3 PPLD Abis LAPD signaling overload

After the overload condition is detected by the BSC,

the alarm message "BTS overload detected" is sent to RC/LMT/ event log file,


the mobile originating traffic is reduced by discarding CHANNEL REQUIREDmessages for the affected BTS based on the current value ofOVLD_LEVEL_UPLINK,

the mobile terminating traffic is reduced by discarding PAGING messages for theaffected BTS based on the current value of OVLD_LEVEL_DOWNLINK.

Note: The starting value for both variables after the first overload message is "10".

4.2.4 PPLD RACH overload

After BTS sends CCCH LOAD INDICATION, no overload actions are performed.


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4.3 Simultaneous BTS and BSC overload

In case of BTS and BSC overload occurring simultaneously, the discarding ofmessages for the BTS affected is based on the maximum value of the variablesrelating to BSC overload and relevant BTS overload (OVLD_LEVEL_UPLINK orOVLD_LEVEL_DOWNLINK).

CHANNEL REQUIRED messages with cause "answer to paging" are treateddifferently:

If BSC is discarding PAGING messages coming via A interface (due to BSC orBTS overload) then CHANNEL REQUIRED messages with cause "answer topaging" are not discarded.

If no PAGING messages coming via A interface are discarded (no BSC or BTS

overload condition), the channel filter applies to all CHANNEL REQUIREDmessages except for the messages relating to emergency calls.

BSC OverloadLevel

BTSOVLD_LEVEL_UPLINK

BTSOVLD_LEVEL_DOWNLINK

Discarded CHANNELREQUIRED msg.

0 0 0 none

0 0 >0 none0 >0 0 all, except emergency calls

0 >0 >0 not "answer to paging"

>0 0 0 not "answer to paging"

>0 0 >0 not "answer to paging"

>0 >0 0 not "answer to paging"

>0 >0 >0 "not answer to paging"


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4.4 Parameters for overload handling

SpecificationName

Object DB Name Range(default)

Meaning

T3122 BSC T3122 0...255 sec

(5)

Timer to bar a new accessfor a certain time afterunsuccessful access (e.g.no SDCCH available)

EN_BSC_OVL_HAND

BSC BSCOVLH TRUE (def.),FALSE

flag to enable/disable BSCoverload handling

EN_MSC_OVL_HAND

BSC MSCOVLH TRUE (def.),FALSE

flag to enable/disable MSCoverload handling

EN_BTS_OVL_HAND

BSC BTSOVLH TRUE (def.),FALSE

flag to enable/disable BTSoverload handling

OVL_START_THRESH

BSC OVLSTTHR 7000... 10000(9500)

threshold for TDPCprocessor (in BSC) overloaddetection: 10000 = 100 %

OVL_END_THRESH

BSC OVLENTHR 7000... 10000(8500)

threshold for TDPCprocessor (in BSC) below

which the sending ofoverload messages isstopped: 10000 = 100 %

CCCH_LOAD_IND_PERIOD

BTS PCCCHLDI 0...255 sec

(255)

period for sending theCCCH load indicationmessage from BTS to BSC

CCCH_LOAD_THRESH

BTS TCCCHLDI 0...100%

(100)

CCCH load threshold, if thisthreshold is exceeded theBTS informs the BSC usingthe CCCH load indication

message


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4.5 Preemption, queuing

This feature allows the assignment of a TCH when, due to the unavailability ofresources, no unused channel can be found within the serving cell. Three mainprocedures are possible:

Preemption, which is a means of providing TCH resources for high priority TCHrequests

Directed Retry, which causes the handover from a SDCCH in one cell to a TCH inanother cell (described in chapter: 'Design of radio cells')

Queuing, which allows the queuing of TCH requests on a per cell and priority

basis

In case of unavailability of resources, two cases can be foreseen: AssignmentRequest and Handover Request.

In case of Assignment Request, Preemption is tried first: BSC looks for a call in thecell with a priority lower than the new TCH request and, then moves the lower prioritycall to another cell performing a Forced Handover (or a Forced Release if HO fails)in order to free the TCH. The freed TCH is then assigned to the new request. If no

call with a lower priority is using a TCH or a call with the same priority or higher isalready waiting (queued) Directed Retry is performed. If still no free TCH is available,the queuing procedure is carried out, storing the TCH request in the cell queue basedon their priority.

In case of Handover Request, Preemption is tried first. If Preemption is notsuccessful, the Directed Retry procedure will be skipped and Queuing is carried out.

Preemption feature gives to the operator the possibility to differentiate

subscriptions per user class (e.g. different contracts for high priority class such uspolice and other emergency body, business people and children)

Queuing and Direct Retry features improve the network access and, in general,network QoS


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Priority Information Element in the following messages from MSC:

Assignment Request

Handover Request

Priority Information Element

Bit 8 Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1

spare PCI PRIORITY LEVEL QA PVI

PCI (preemption capability indicator)

it indicates if the preemption shall be applied or not.

PRIORITY LEVEL

it contains the priority of the call; GSM supports the prioritization of calls using14 priority levels (1 is the highest and 14 is lowest priority)

QA (queuing allowed indicator)

it is used to perform the queuing procedure.

PVI (preemption vulnerability indicator)

it indicates if this connection might be preempted by another allocation request.

In GSM different priorities and access types are supported via the Priority InformationElement being (optionally) conveyed in the ASS REQ or HO REQ message. Thepriority information element contains the following entries:

The Preemptive Capability Indicator (PCI) applies to the allocation of resources for anevent (e.g. ASS REQ, HO REQ) and indicates whether the event is able to trigger thepreemption of radio resources.

The Preemptive Vulnerability Indicator (PVI) applies for the entire duration of aconnection and indicates whether the connection may become a target ofpreemption.

The Queuing Allowed Indicator (QA) is used to decide on a per call basis if queuingshall be applied or not.

The Priority Level (PL) is subdivided in 14 different levels, Priority Level 1 being thehighest value.


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The entire procedure is shown in the following Figure. These flowchart basicallyindicates which algorithms and which checks are necessary in which network entity(BTS, BSC or MSC) and the message flow that has to be conveyed between the

networks entities. It can be seen immediately that, in the best case, an assignment ofa TCH in a neighbor cell takes place.


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Flowchart of Preemption

ASS_REQ / HO_REQ (no TCH available)

Set PCI=0, PVI=0,

QA=1, PL=14

N

PCI = 1 ?

N

Y

Other queued calls with

PCI=1 and higher or

equal priority ?

Y

Other active call with

PVI=1 and lower PL

N

Successful ?Forced Release of the

lower PL call

BSC assigns the free TCH

Y

N

Y

Y

N

Directed Retry

Procedure

Does the message

contain the priority I.E.?

Send Forced Handover

for the lower PL call

Fig. 3 Flowchart of preemption


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Parameters and Timers for Preemption and Queuing

EN_PREEMPTION

Object DB Name Range Step Size Unit

BTS EPRE ENABLEDDISABLED

-

Enable Preemption is used to enable/disable the preemption feature.

EN_QUEUEING


BTS EQ ENABLEDDISABLED

-

Enable Queuing is used to enable/disable the queuing feature.

QUEUEING_LENGTH


BTS QL 1...100 1

Queuing Length defines the maximum number of TCH requests hat can be queued inthe cell.


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T11


BSC BSCT11 unit * (0...255) 1 MS100 = 100 msecHLFSEC = 0.5 sec

SEC5 = 5 sec

Purpose: maximum queuing time for ASS_REQ

Start: ASS_REQ is queued, BSC sends QUEUE_IND to MSC

Stop: TCH becomes free for assignment or the request is replaced by another request with higher priority.

Action expiry: remove ASS_REQ from queue, send CLEAR_REQ to MSC

Default: HLFSEC-16

Tqho


BSC BSCTQHO unit * (0...255) 1 MS100 = 100 msecHLFSEC = 0.5 sec

SEC5 = 5 sec

Purpose: maximum queuing time for HO_REQ

Start: HO_REQ is queued, BSC sends QUEUE_IND to MSC

Stop: TCH becomes free for assignment or the request is replaced by another request with higher priority.

Action expiry: remove HO_REQ from queue, send HO_FAIL to MSC

Default: HLFSEC-20

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